Molybdenum is an essential metal in enzymes involved in purine metabolism, sulfur metabolism, the assimilation of inorganic nitrogen, and interconversions of inorganic nitrogen in the biosphere. Exposure to excess molybdenum has been linked with gout and atherosclerosis, and molybdenum deficiency interferes with normal sulfur metabolism. The molecular structures of the molybdo-enzymes are unknown, however, there is evidence that all possess a common molybdenum cofactor. The nature of the molybdenum centers of the enzymes and of the cofactor are also unknown, but it is generally thought that one or more sulfur atoms are coordinated to the molybdenum atoms. The objectives of this research project are to investigate structural models for molybdenum centers by an integrated series of chemical, physical and x-ray structural studies on well-defined molybdenum compounds. Special emphasis will be given to compounds which mimic the electron paramagnetic resonance (EPR) spectra of molybdo-enzymes, compounds which mimic the reactions of molybdo-enzymes, and compounds with sulfur-containing ligands.